The traditional methods for capillary gas chromatography involve injecting a sample for analysis into a carrier gas. The sample is carried by the carrier gas along a capillary having an inner wall onto which the sample is partitioned, leading to a slower migration of the analyte vapors relative to the carrier gas. The partitioning involves a portion of the sample (which can be referred to as the partitioned portion) that bonds to the capillary and that is then released in a continuous process on the molecular scale. In the case where the capillary is coated with a fluid film, which is more common, the bonding occurs by absorption in the fluid film. Alternatively, the bonding can take place by adsorption on a solid surface.
The migration rate (v) of a given analyte and the flow rate (u) of the carrier gas are related by: v=pu, where p is the retention ratio. p is the probability of an analyte to be in the carrier gas (1−p being the probability of absorption). The retention ratio typically varies with the nature of the analyte, so that each analyte has a characteristic migration rate in a given sample. To facilitate understanding, reference is made to FIGS. 1A and 1B which schematically illustrate the concentrations of two different analytes migrating along the length of a capillary at two different moments.
Each analyte can thus bethought of as travelling in the form of a distinct packet, or zone of higher concentration, having a characteristic migration rate. Typically, each packet has a sharp zone distribution at the inlet of the capillary, and this zone gradually broadens as the packet travels along the capillary. The zones associated to different analytes also become progressively more spaced due to the characteristic migration rates of the analytes, which then make them more distinct.
A change in the response of a suitable sensor (such as a thermal conductivity sensor, for instance) placed at the exit of the capillary can indicate the passage of an analyte. The characteristics of the capillary and the flow rate of the carrier gas being known, a detection at a given moment can be associated with a migration rate characteristic of a specific analyte.
U.S. Pat. No. 7,403,673, the contents of which being incorporated herein by reference, teaches a new approach to chemical sensors. This approach involves guiding light in a birefringent optical waveguide that has a light propagation volume (such as a core) positioned adjacent to a capillary. The propagation volume and the capillary are close enough so that an analyte absorbed in the stationary phase can interact with the evanescent field of the guided light by altering the polarization state of the light. Information on the fluid to analyze is obtained from the detected variations in the polarization state of the light by measuring the light power transmitted through an optical polarizer placed at the output of the waveguide. This approach involves using a birefringent optical waveguide that has two different refractive indexes defining the birefringence B and the polarization beat length Lb. For a given light wavelength λ, both parameters are related by:
      L    b    =            λ      B        .  
The beat length Lb is the distance along the birefringent optical waveguide that corresponds to a phase shift of 2π between the two polarization modes of the light, and it is thus the length along the waveguide for which a polarization state of the light is recovered.
In the case of an optical fiber polarimetric chemical sensor where, for instance, linearly-polarized light is injected with its polarization direction parallel to one of the polarization axes of the optical waveguide, the presence of locally absorbed vapor in the capillary, which is adjacent to the propagation volume, transfers some of the light to the other polarization axis, and can thus be said to constitute a coupling point between the polarization axes. The new polarization state, which can be elliptical for instance, then evolves towards the optical fiber output where it can be analyzed with a polarizer. When a single light wavelength is used, as analytes are moving at speed (migration rate) v and as polarization states reproduce themselves at each distance equal to the beat length Lb, the light power transmitted through an output polarizer will oscillate at an oscillation frequency, or beat frequency fb given by:
      v          L      b        .
The transmittance of the optical waveguide, including the output polarizer, can be given with a good approximation by:
            I      ⁡              (        t        )                    I      0        =            1      2        -                  ∑                  j          =          1                N            ⁢                          ⁢                        κ          j                ⁢                  cos          ⁡                      (                                                                                2                    ⁢                    π                                                        L                    b                                                  ⁢                                  v                  j                                            +                              φ                j                                      )                              where φ is a phase term that can be discarded. The summation is performed over the analytes present in the sample fluid. The Fourier transform of the detected signal I(t) shows spectral peaks having locations that correspond to the specific migration rates of the analytes.
In the above equation κj is the strength of the polarization mode coupling caused by the presence of the analyte j. This parameter is related to the concentration of each analyte and to its distribution in the capillary fiber. It will be understood that for very small quantity of analytes the mode coupling can be very small, so that the amplitude of the signal detected at the oscillation frequency fb can be too weak to be detected in the Fourier spectrum of I(t).
As a result there remains room for improvements, particularly for increasing the sensitivity of such chemical sensors.